In the medical field, it is often necessary to measure the respired gas volume in a person. This gas volume is obtained by measuring the velocity of gas flow, breath by breath, and integrating the velocity over a defined period of time.
There are many methods of measuring the gas velocity. One such method involves the use of a Fleisch type pneumotachometer (flowmeter) that was invented in the 1920's. A Fleisch pneumotachometer includes a tube, two sets of screens to linearize the turbulent inconming gas flow to establish a region of laminar flow, and a pressure sensing transducer located between the screens. Since the pressure drop .DELTA.p is proportional to he gas velocity, the gas velocity can be determined by measuring .DELTA.p across the pressure sensing transducer. However, the Fleisch pneumotachometer works well only if the laminar regime is maintained. Turbulence causes .DELTA.p to vary nonlinearly with gas flow and thus affects the accuracy of the gas velocity determined using a Fleisch pneumotachometer. Accumulation of saliva and/or effluent inside the tube from the person's lungs, particularly at the screens, will cause turbulence to occur. In general, to overcome the nonlinear effects, different diameter tubes are used to cover a range of up to 20 liters per second (l/sec).
Since the availability of lead titanate zirconate (PZT) piezoelectric material, ultrasonic flowmeters are now available, and are in common use for various applications. PZT is a piezoelectric material, which can be used to generate ultrasonic waves at different frequencies. In these pneumotachometers, the velocity of gas flow is determined by measuring the transit time of an ultrasound wave through a gas volume. However, accumulation of spit and other pulmonary effluents from a person's lungs tend to block the wells in front of the transducers, making the pneumotachometer unreliable in those situations. Also, since such a pneumotachometer often uses small and single transducers, there is always a likelihood that the transducers could be totally blocked, causing the transducer to stop acquiring data. It would therefore be very beneficial to the art if the transducer output could be desensitized to such dropouts of the signal acquisition.
A solution, which would provide an accurate and reliable pneumotachometer for determining the velocity of a fluid flow, has been long sought but has eluded those skilled in the art.